War Crimes At 20,000 Feet?

Inspired by a review of Martin van Creveld's The Age of Airpower, Gulliver poses a provocative question:

How is it that William Calley is so reviled for the killing of 22 Vietnamese civilians while Harry Truman is largely forgiven for the killing of perhaps 200,000 Japanese civilians? Why is tactical atrocity punished while strategic atrocity is applauded?